Naltrexone (1) is a narcotic analgesic:
Nalbuphine (2) and nalmefene (3) are structurally similar compounds:
Oxilorphan (4) and butorphanol (5) are also similar but there is no ether linkage between the so-called A and C rings:
Diprenorphine (6) and buprenorphine (7) contain an ethyl bridge on the C ring:

U.S. Pat. No. 3,332,950 discloses methods for preparing this type of compound. In a first method, Naltrexone is prepared from noroxymorphone in four steps. The method employs a hazardous metal hydride reagent in order to reduce the condensation product formed between a protected noroxymorphone and cyclopropylcarbonyl chloride. In order to prevent side reactions occurring at the ketone functional group, the method incorporates protection and deprotection steps. The inventors believe that this type of process afford yields of approximately 33% naltrexone starting from noroxymorphone hydrochloride. In a second method, naltrexone is prepared by the direct coupling of cyclopropylmethylbromide and noroxymorphone in dimethylformamide. The method employs high temperatures (70° C.) and prolonged reaction times (7 days) yet still only achieves a 60% theoretical yield.